Hydrogen has been endorsed by numerous world leaders and decision makers in both public and private sectors and hailed as the key to a clean energy future. Many believe that the future alternative to current fossil-based economy will involve hydrogen—used as the primary energy carrier in all energy sectors. In that regard, hydrogen will be utilized much like electricity that is presently the defacto energy carrier in much of the world.
Hydrogen and electricity are complementary energy carriers and together they can create an integrated energy system based on distributed power generation and use. Hydrogen and electricity are interchangeable using a fuel cell (to convert hydrogen to electricity) or an electrolyzer (for converting electricity to hydrogen). A regenerative fuel cell works either way, converting hydrogen to electricity and vice versa.
The advantages of using fuel cells for power generation include: 1) ability to convert fuel into electricity directly; and 2) being an electrochemical device, they are more efficient than the Carnot cycle based energy conversion devices such as internal combustion engines (ICE).
As noted above, hydrogen can be used in fuel cells to power electric motors. Furthermore, hydrogen, in combination with proton exchange membrane fuel cells (PEMFC), can replace rechargeable batteries as the power source of choice in certain applications, such as military and other applications requiring portable power.
Presently, rechargeable power sources are used for cell phones, PDAs, and laptop computers, among others. However, the majority of military applications, particularly those involving soldier-portable devices still utilize standard, non-rechargeable primary batteries. The reason for this is that primary batteries offer long shelf life, robustness, and ease-of-use.
Secondary (rechargeable) batteries have improved over the years, but an alkaline primary battery still delivers 50% more power than lithium-ion, one of the highest energy density secondary batteries in existence. The primary lithium battery used in cameras provides more than three times the energy of a secondary lithium-ion battery of same size. Moreover, rechargeable batteries are vulnerable to the elements as well as extreme temperatures, humidity, salt, and other exposure. While recent innovations in the technologies used in rechargeable batteries has reduced these risks, rechargeable batteries are not yet as powerful, rugged or reliable as primary batteries.
Despite their dependability and superior capacity, the one-time use constraint of primary batteries increases their cost to over thirty times that of rechargeable batteries (mainly because new batteries are constantly required to be replaced). Moreover, the logistics of getting new batteries delivered to military field units can be challenging or even impossible. Soldiers are often required to carry up to 30 lbs of batteries in the field to power their electronic gear. Rechargeable batteries would obviously be a much lighter alternative, but their limitations due to exposure make them too risky for many defense applications, even though they are still used in military training exercises.
Compounds represented by the empirical formula BxNxHy have been known and used as high capacity hydrogen carriers. Generation systems based on such carriers can provide hydrogen storage for hydrogen on-demand applications. However, the hydrogen is often too impure for important applications, the process yield is low, the process requires a high temperature (e.g. greater than 100° C.) for reasonable yield, and/or environmentally harmful materials are required.
For example, U.S. Pat. No. 4,157,927 to Chew et al. discloses amine boranes and their derivatives mixed with heat producing compounds such as lithium aluminum hydride or a mixture, such as NaBH4/Fe2O3 as hydrogen generating formulations. The mixed powder is then pressed into pellets and ignited to generate hydrogen or deuterium. The formulations disclosed do not produce the ultra high purity hydrogen gas required by PEMFC and other demanding applications.
U.S. Pat. No. 4,381,206 to Grant et al. discloses an amine borane gas generating system comprising hydrazine bis-borane or its deuterated derivatives in the form of a pellet, which serves as the thermal initiator for further thermal decomposition. An all amine borane gas generator system which consists of N2H4.2BH3 and H2B(NH3)2BH4 provides hydrogen from a self-sustaining reaction after the self-sustaining reaction is initiated by a heat source such as a nichrome wire. Again, formulations disclosed by Grant do not produce the ultra high purity hydrogen gas required for the PEMFC applications.
Ammonia borane (AB) complex has the highest hydrogen content (about 19.6 wt %)—highest amongst all amine boranes with a system-level H2 energy storage density of about 2.74 kWh/L vs. 2.36 kWh/L for liquid hydrogen. At near room temperatures and atmospheric pressure, AB is a white crystalline solid, and is stable in both water and ambient air.
Thermolysis has been used as a method of choice to generate hydrogen from AB complexes. The drawbacks of thermolysis for hydrogen release are as follows:
Ammonia borane pyrolysis begins at temperatures below 140° C. To release substantially all of the hydrogen contained in ammonia borane complex, however, temperatures above 500° C. are required. The overall process is exothermic, but requires heat to be added to activate the AB complex. The overall thermolytic reaction can be written as follows:NH3BH3+Heat→BN+3H2 
In practice, thermolysis of AB complex involves competing reactions leading to the formation of certain undesirable by-products. For example, FTIR analysis of the evolved gases from the thermolysis of AB complex has shown that monomeric aminoborane (BH2NH2), borazine, and diborane is also produced. The aminoborane comprises poly-(aminoborane), (BH2NH2)n.
Poly-(aminoborane), the inorganic analog of polyethylene, is a nonvolatile white solid. Volatile compounds are undesirable impurities that make hydrogen from direct thermolysis of AB complex unfit for PEMFC applications. Furthermore, formation of these undesirable compounds lowers the yield of H2 from direct thermolysis of AB complex.
Because direct high temperature thermolysis of amine borane complexes, in general, and AB complex, in particular, generates low yield of inferior quality hydrogen contaminated with volatile pyrolysis products, it is highly desirable to find new energetically self-sufficient processes that can generate ultra high purity H2 at low, near ambient temperatures.
Hydrolytic or methanolic cleavage of amine borane complexes provides 3 moles of hydrogen per mole of AB complex. Although this process has been used in the field of modern synthetic organic chemistry and for pharmaceutical applications, only recently has it been applied as a way of utilizing AB complex for the storage of H2:NH3BH3+3H2O→NH3+H3BO3+3H2 NH3BH3+3CH3OH→NH3+B(OCH3)3+3H2 
As mentioned above, AB complex is a stable adduct. Therefore, the above hydrogen generating reactions have involved harsh acidic condition (e.g. refluxing in aqueous HCl) or the use of heterogeneous catalysts based on palladium or nickel.
Using methanol as a reagent, such as in the reaction above, has a major drawback as it increases the overall weight of the AB-based hydrogen generator, thus reducing the overall specific H2 energy storage density of the hydrogen generator.
Published U.S. Application No. 20050180916 to Autrey et al. discloses ammonia borane deposited onto a support or scaffolding material at a 1:1 weight ratio. Supports are porous materials such as mesoporous silica. Autrey also discloses a transition metal catalyst and a carbon support. This material exhibits hydrogen release rate that is about one order of magnitude greater than that from the neat compound. In addition, the disclosed temperature for hydrogen release is reduced to about 85° C.
Thus, all known methods for low temperature hydrogen release from an AB complex and related materials have significant shortcomings. Thermolytic dehydrogenation requires a relatively high temperature and generates a significant concentration of by-product impurities which make this process unfit for certain important applications. Although Autrey et al. discloses dehydrogenation of AB complex at about 85° C., a special or complicated support structure is required and no more than two moles of hydrogen are generated. Hydrolytic or related cleavage processes require harsh chemicals, such as HCl in reflux temperatures. Therefore what is needed is an energetically, self-sustaining method and apparatus for hydrolytic and/or thermolytic dehydrogenation of amine borane complexes, in a hydrogen generator for portable, on-board and off-board power generation via PEMFC and ICE.